Green’s Dictionary of Slang

a general excl. of exultation, excitement.

1905		Sporting Times 11 Mar. 1/5: It is said to be the desire of His Majesty that the Prince of Wales should ‘see every part of the Empire.’ Banzai! [Ibid.] 13 May 1/3: ‘Total £257 8s. Banzai!’ No false Japanese joyousness illumined the features of the victim, however.
1905		Sun. Times (Perth) 22 Jan. 4/7: Let us raise a mighty Banzai! for the town of many twins.
1909		Wodehouse Mike [ebook] ‘Doctor Oakes thinks he will be back in school on Tuesday.’ ‘Banzai!’ said Burgess, feeling that life was good.
1911		T.A. Dorgan ‘Daffydills’ in Atlanta Constitution (GA) 9 Sept. 10: Percy the stalwart deckhand [...] grabbed fair Mabel round the lunch basket. Banzai, banzai, huroo, hurraw.
1912		Bulletin (Sydney) 12 Dec. 36/2: Good luck to our legislation, / And the Will Watt Cabinet! / Give thanks to the British Nation, / Huzza for the National Debt! / Banzai for the cash we borrow, / In the good old Tory way, / I never find cause for sorrow, / For our splendid loans, hooray!
1914		Bulletin (Sydney) 23 July 7/4: If they did know they would shout ‘Banzai !’ as well as a drink or two for the party system.
1926		‘Ford Madox Ford’ Man Could Stand Up 159: ‘Shout Banzai! men,’ he said.
1959		(con. 1940s) ‘David Forrest’ Last Blue Sea 160: Banzai! They screamed [...] running hard at him with their bayonets.
1969		P. Boyle All Looks Yellow to the Jaundiced Eye 104: ‘Banzai!’ I called out when she finished.